This invention relates to an apparatus for detecting a string stop position or positions, which apparatus detects position data of a string or strings stopped or pressed by a finger or fingers in string instruments such as a guitar having a tightened string or strings and a plurality of frets for determining a sound pitch.
Such an apparatus for detecting string stop positions is normally used where detected signals are output to a synthesizer so as to play with the corresponding musical scale. U.S. Pat. No. 4,468,997 or Japanese Application Serial No. 59-21524 is (Publication No. 59-176783) known as the prior art.
Recently, a MIDI standard has been established internationally by centering around keyed instruments and it has tried to extend the standard to the field of string instruments. However, in the case of the string instruments, there is a particular difficulty of how to detect played notes (pitch of name). Namely, in the case of the keyed instruments, a switch is arranged at each key so that the played note or notes may be detected by means of on-off operations of the switch. However, since such string instruments are not provided with keys normally, it is very difficult to detect the played notes in comparison with keyed instruments. Therefore, many kinds of means for detecting played notes have been proposed for string instruments. For example, there is an invention desclosed in Japanese Application Ser. No. 59-21524 (publication No. 59-176783). This invention may detect a string stop position by utilizing electric resistance of the string. In detail, when the string is stopped, the string contacts a pair of frets located on both of the sides of the stopped position thereof. When an electric current flows through the string, there occurs a difference in electric potential in accordance with the value of resistance of the string. Thus, the stopped position of the string may be detected by detecting the difference of the electric potential between the frets.
However, according to the above-mentioned conventional detecting means, there are problems as follows:
1. The strings, which are normally used for, for instance, an electric guitar, are made of steel and each of the electric resistances of the strings is 10-20.times.10.sup.-8 .OMEGA.m. Therefore, the defference of the electric potential occurred in the string is considerably small and sometimes it is substantially equal to the level of noise.
2. To evade a mistake or wrong operation due to the small difference of the electric potential, it is necessary to increase the electric current flowing through the string so that the value of the electric potential to be detected may be much greater than that of the noise level.
However, it is impossible to flow such a great electric current in the conventional apparatus which utilizes an IC as electric current supply means for the strings.